The development of fiber optics has created a need for high performance new nonlinear optical materials which can be used in optoelectronic devices for integrated optical systems. Input devices must be fabricated to allow the conversion of the photonic to electronic signal for telephones, TV's, VCR's and personal computers. The photonic input is generated by lasers which are already used in Supermarket Barcode Readers, Compact Disc Players, Fax machines, or Laser Printers. The photonic signal must be demultiplexed, split, switched and routed. The search is on for optical equivalents of electrical connectors, resistors, capacitors, transistors, dielectrics and transformers. Waveguides and optical switches are examples of such optical equivalents.
The unusual optical properties of certain polymers make them useful in the field of optical computing, or optoelectronics, which is aimed at replacing electronic microchips that operate using electricity with optoelectronic devices that would operate with pure light at much greater switching speeds. For example, a standard silicon microchip will work at a speed in the range of microseconds, while a gallium arsenide microchip has switching speeds on the order of nanoseconds, and optoelectronic materials made out of conductive polymers generally have a switching speed in the picosecond range.
Research and engineering personnel are testing conductive materials as optical switches, the optical counterparts of transistors. To date, polydiacetylene has appeared to create a device that allows switching on a picosecond time scale. A desired material would have an exceptionally high optical nonlinearity so that switching can be done with inexpensive, low-intensity die lasers. Most of the tested optical switches recently formed have required a much more intense light to function properly. Unfortunately, the above-mentioned polydiacetylene is a disadvantageous material for optical devices because it absorbs light as strongly as it distorts it. It is the distortion enabled by the chemical structure of the conductive polymers which interferes with the propagation of light through the material, thus forcing the speed of light to vary with the intensity of the light. A search is currently on to look for a conductive polymer which will combine high nonlinearity with good transparency.
Nonlinear optical devices are devices based on a class of optical effects that result from the interaction of electromagnetic radiation from lasers with nonlinear materials. Nonlinear means that the effect depends on the intensity of the light. Nonlinear effects are due to the nonlinear contribution to the polarization of the medium, which can be expressed as a power series expansion in the incident electric field E by the following equation EQU P=E.sub.0 (X.sup.(1) E+X.sup.(2) E.sup.2 +X.sup.(3) E.sup.3 +. . . )
wherein X.sup.(1) is the linear, and X.sup.(2) and X.sup.(3) are second- and third-order susceptibilities, respectively. X.sup.(2) has a nonzero value only in materials that do not possess inversion symmetry, but X.sup.(3) is usually nonzero in all materials. Four-wave mixing and phase conjugation are examples of third-order, X.sup.(3) nonlinear effects. Optical bistability, which can also be expressed as a X.sup.(3) mechanism, occurs when the nonlinearity is coupled with feedback.
Nonlinear optical devices can be classified roughly into two categories: (1) X.sup.(2) devices that generate light at new frequencies and (2) X.sup.(3) devices that process optical signals. The second category contains waveguides, four-wave-mixing beam deflectors, phase-conjugate mirrors, etalon switches and logic devices. Current emphasis is being placed on waveguide applications.
Four-wave-mixing beam deflectors have been found to be especially useful for optical techniques for intracomputer communications and have become increasingly popular because of the limit imposed by the electronic interconnections on the scale-downs of computer circuits. One method that has been assessed for optical interconnection is the use of holographic elements to diffract the light to the desired locations. Holographic arrays may someday replace the huge number of wires or buses that are used inside the computer to transfer information. However, conventional holograms are not programmable and would have to be replaced every time a different set of pixels was to be accessed.
Four-wave mixing is one of the avenues that has been investigated to perform real-time programmable holography. Dynamic gratings that are generated by four-wave mixing may be able to deflect light originating from a source on a very large-scale integrated circuit (VLSI) chip to a spot on the same chip or to another VLSI chip. If the light is desired to illuminate a different spot, the direction of the grating must be changed; this can be accomplished by changing the angle at which the two "write" beams intersect.
Nonlinear optics is a field of study concerned with the interaction of electromagnetic radiation and matter in which the matter responds in a nonlinear manner to the incident radiation fields. The nonlinear response can result in intensity-dependent variation of the propagation characteristics of the radiation fields or in the creation of radiation fields that propagate at new frequencies or in new directions. Nonlinear effects can take place in solids, liquids, gases, and plasmas, and may involve one or more electromagnetic fields as well as internal excitations of the medium. The wavelength range of interest generally coincides with the spectrum covered by lasers, extending from the far infrared to the vacuum ultravoilet, but some nonlinear interactions have been observed at wavelengths extending from the microwave to the x-ray ranges. Historically, nonlinear optics precedes the laser, but most of the work done in the field has made use of the high powers available from lasers.
Nonlinear effects of various types are observed at sufficiently high light intensities in all materials. It is convenient to characterize the response of the medium mathematically by expanding it in a power series, as described hereinabove, in the electric and magnetic fields of the incident optical waves. The linear terms in such an expansion give rise to the linear index of refraction, linear absorption, and the magnetic permeability of the medium, while the higher-order terms give rise to nonlinear effects.
Certain requirements for nonlinear optical materials have been promulgated which indicate the characteristics of an ideal nonlinear optical material are as follows:
1. large nonlinear optical response, PA1 2. low switching energy, PA1 3. rapid switching times, PA1 4. nondispersive, PA1 5. mechanically tough and formable, PA1 6. high damage thresholds, PA1 7. formable into thin films and coatings, PA1 8. easy to manufacture, PA1 9. useful at high and low temperatures, and PA1 10. immune to corrosive and oxidative environment.
Polymeric materials are currently being investigated for many different applications. Conductive polymers are currently being used for plastic batteries, anti-static coatings, and heat reflective coatings and electromagnetic shields. Conductive polymeric fiber clothing may be used by workers to ward off static during the production of semiconductors, complicated parts and during medical operations. It has been speculated that conductive polymeric paint may be used to coat computer cabinets or even entire buildings to keep electromagnetic radiation from leaking into the atmosphere. There have been many years of research which have been directed towards the optimization of polyacetylene as a conductive polymer. Plain polyacetylene does not conduct electricity very well unless it is doped to enhance the conductivity by several orders of magnitude.
Problems posed by conductive polymer materials include poor stability in air and a stubborn resistance to being processed due to their insolubility in common solvents. The stability problem arises in conductive polymers because the chain of carbon atoms are connected by alternating single and double bonds, which prove to be extremely weak conductors. When the materials are oxidized, their conductivity can rise by many orders of magnitude. However, therein lies the problem; the gaps left by the lost electrons provide a pathway for electronic charges to be conducted down the polymeric chain. At the same time, this same property makes the conductive polymer highly reactive with water, such as the humidity in air, which is increased at high temperatures. Generally, conductive polymers would be exposed to high temperatures during their operation, and this would increase the reactivity of the conductive polymer with the humidity in the air.
Furthermore, conductive polymeric materials exhibit a resistance to processing which stems from the fact that the polymers form rigid, tightly packed chains. While the tight packing of the chains is essential for electric charges to be able to jump from one molecule to the next as the current moves through the polymer, this also means that the polymer as a whole is a hard, insoluble mass because the polymer chains resist intermixing with solvent molecules. This tends to render the polymers unprocessible and essentially unformable into fibers, thin films and coatings.
As researchers gained knowledge about the conductive polymers during the 1980's, they solved the stability problem by incorporating less reactive atoms such as sulfur, nitrogen and oxygen into the polymeric backbone. For example, a particularly successful polymer is polypyrrole, a chain of five member rings, each of which contains a nitrogen atom. Several Japanese firms, including Nippon Electric, now sell high frequency capacitors containing polypyrrole as the solid electrolyte. Structurally analogous polypyrrole compounds having sulfur and oxygen atoms instead of the nitrogen, polythiophene and polyfuran, have also proved to be stable and conductive, although they were still presented with the processibility problem. Different side chains to the basic polymers were tested by trial and error to attack the processibility problem.
Therefore, it is an object of the present invention to provide a conductive polymer which is stable in high temperatures, has a high molecular weight, and is easily processible to form fibers, thin films, coatings, or bulk materials so that articles of manufacture may be easily fabricated.
Traditionally, it had been reported by Luneva, et al. in 1968 that heating diphenyldiethynylsilane afforded a low molecular weight (about 2700 to 5000), red, soluble polymer which was claimed to have a straight chain structure containing diacetylenic groupings. Diphenyldiethynylsilane was heated at 180.degree. to 200.degree. to form a reddish-brown polymeric solid which was soluble in benzene or toluene. This conventional material experienced problems due to its low molecular weight.
Improved polymeric materials which increase the conjugation of the polyenes, such as polyacetylene and polydiacetylene were described by Kusumoto and Hiyama in 1988. Their materials were disclosed for conductivity applications. In spite of the promising conductive properties, the instability of the polymers to atmospheric oxygen severely limited their use. Substitution of the polymers endowed a remarkable stability at the total expense of the conductivity, however. Therefore, Kusumoto et al. attempted to synthesize a soluble, air-stable and conductive polymer by cyclopolymerization of monomers containing two ethynyldimethylsilyl groups. The polymerization was catalyzed with a WCl.sub.6 or MoCl.sub.5 catalyst which rendered polymer, soluble in common organic solvents. The polymers were doped to improve their conductivity. Their materials were of low molecular weights, from about 2800 to 3900, and colored yellow or red to brown. The results do not include a blue or violet polymer.
These examples of previous attempts to produce a stable, easily processible conductive polymer when doped, and to produce a high quality nonlinear optical polymeric material when left undoped are described above. As can be seen from their experimental data, they may have solved one or two of the problems, but they did not solve all of the problems.
Therefore, it is a primary object of the present invention to provide a polymeric material in accordance with the present invention which produces a blue or violet polymer which exhibits highly enhanced nonlinear optical properties. This polymeric material should be easily processible, have a high molecular weight and should be stable in high temperatures.
It is yet another object of the present invention to provide a conductive polymeric material which exhibits an enhanced conductivity in the range of 10.sup.-2 to 10.sup.1 S/cm in a material which is easily processible, easy to dope and stable in air.